1. Field of the Invention
This invention relates to metal hydrides and in particular to processes involving such hydrides.
2. Art Background
Metal hydrides are used in a variety of industrial applications. Although there are many such applications, possibly the most prominent is the use of metal hydrides in batteries. For example, secondary nickel-metal hydride batteries employ lanthanum nickel hydride (or alloy modifications) or other intermetallic hydrides in the negative electrode. A variety of other uses involving energy storage and transfer have been described. Irrespective of the application, a crucial step in preparation is activation of the hydrideable element, alloy, intermetallic compound, or mixture thereof (referred to generally herein as "metals"). Activation increases the rate at which the metal reacts with hydrogen or the extent to which hydrogen is incorporated into the intermetallic, thus making the metal useful for energy storage and energy transfer applications.
Activation is believed to result from 1) removal of reducible surface oxides which tend to interfere with the functioning of the material in the ultimate desired application; 2) reduction of particle size resulting from an increase in volume, which fractures the metal particles; and (3) changes in the chemical composition and/or structure of the metal or the surface of the metal. Thus, activation, it is believed, increases the surface area and perhaps alters the chemical composition and/or structure of the metal and/or the surface of the metal, any combination of which may lead to higher rates of reaction with hydrogen, enhancing the operation of the material for applications such as batteries or hydrogen storage. Metals with this enhanced chemical reactivity toward hydrogen are referred to as activated.
Methods for activating metal hydrides include: 1) hydriding with hydrogen gas at high temperature and/or high pressure; 2) hydriding with chemical hydriding reagents;. 3) etching with reagents such as aqueous hydrofluoric acid or hot potassium hydroxide; 4) electrochemical anodic oxidation; and 5) conventional battery cycling of metal hydride electrodes. Such methods can require relatively large expenditures for suitable equipment and vary in their effectiveness, depending upon the metal being activated. Thus, alternatives would be quite desirable.